No taking two Slots. If you want to gimb yourself to get several half stacking bounses, go for it.
Anything stacks to infinity and beyond, except something like miss chance and crit range (It eventually reaches a point where buying more gives nothing).
Speciality is described in the jutsu chart.
Grade 4 is only avalible for people who are allowed to take it.
Yes Revisions will always be made
People who have made without accounting for stacking rule can in most cases get a revision.